Measure carefully with calipers to see what you actually need. Then look up Agena Astro and see if they have something. If that doesn't work then the expensive alternative is Precise Parts or give the scope vendor a call and see if they have any ideas. I'm not a fan of using a reducer or flattener intended for one telescope on another. I'd check with SV and see what they say about that idea as well (and they might sell adapters).